Microorganisms, including bacteria, are ubiquitous in nature and can have profound negative effects on oil and natural gas recovery. Bacterial fouling of the water needed to hydrofracture (“frac”) reservoir rock or to “water-flood,” to increase production of oil and gas, can contaminate or “sour” the reservoir by producing hydrogen sulfide (H2S). This decreases the value of the product and can make marginal wells unprofitable. Sulfate reducing bacteria (SRB) produce toxic, flammable H2S, which shortens the lifetime and lowers the reliability of any piping and tankage, and introduces additional safety risks from drill rig to refinery. Acid producing bacteria (APB) produce acids, including sulfuric acid, which lead to additional corrosion.
Bacterial fouling leads to serious problems in the oil and gas industry. Bacterially-evolved hydrogen sulfide sours petroleum reservoirs, elevating risk and devaluing the product, while bacterial production of iron sulfide creates black powder accumulation, causing pipeline blockages. Microbially-influenced corrosion attacks the whole system, from fracture tank to refinery, and degrades fracture fluid additives.
In Barnett Shale operations in Texas, water is typically stored in large ponds which are open to the atmosphere prior to the start of fracturing work, allowing the water to become heavily contaminated with bacteria. In addition, bacteria become established in biofilms near the wellbore during shut-in of the well.
The Barnett Shale formation's low permeability requires the use of large-volume hydraulic fracturing technologies to enhance gas production. Other shale formations, such as the Marcellus in the eastern U.S., also require hydraulic fracturing. In a typical “frac” operation, water is collected in portable tanks or large, purpose-dug ponds from a variety of sources, including water wells pumping from aquifers, chlorinated city water supplies, and ponds, rivers, and lakes. Each of these water sources has some level of innate indigenous bacterial contamination that continues growing during the collection reservoirs' exposure to the atmosphere.
Hydrofracturing (“fracing”) and “water flooding” is heavily dependent on the availability of water, and a typical horizontal “frac” operation requires one to five million gallons of water. The water is pumped into a production well at very high rates (one to over two hundred gallons per minute (gpm). Droughts such as that affecting the Barnett shale operational area have been common over the past several years. During times of drought, water recovered from previous hydro-fracture operations (“flow-back” or “produced” water) is reused, and mixed with “fresh” water in holding ponds or tanks. This reused water introduces elevated bacterial fouling concentrations and solids loadings. Even in times when no drought exists, the universal use of flow-back water in all “frac” operations is utilized to mitigate the expense and environmental harm done in removing and disposing the highly contaminated waste water and is increasingly being required by regulation.
To counter bacterial fouling and reservoir souring, chemical biocides, commonly hypochlorite bleach, are applied to the fracture water. The cost of the biocide treatment for a single typical “frac” operation can be as much as $50,000. Additionally, the design of recovery systems with sour service alloys, thicker pipe, and heavier valves leads to increases in capital expense.
The scale of the problem is enormous. The Barnett Shale underground natural gas formation extends over 5,000 square miles in north central Texas. A total of 6,519 gas wells with a further 4,051 permitted locations existed as of Aug. 15, 2007. Wells are being drilled within populated areas, such as the Dallas-Fort Worth city limits, where it is vital to minimize risk and environmental impact. The petroleum industry currently spends $2 billion on biocides annually. Broad spectrum biocides require the additional expenditures associated with regulatory compliance. These biocides may remain in the water when it is pumped out of the well, creating waste handling and disposal problems. Understandably, biocide usage in the petroleum industry is facing growing regulatory resistance because of the negative impact on the environment and associated health risks.
As well as requiring enormous expenditures, biocides are not sufficiently effective. Any bacteria that are endemic or are introduced into the formation encounter favorable growth temperatures and conditions during the “frac” and flooding operations, as the large volumes of water pumped downhole result in near wellbore cooling. Wells may be shut in following the operation while surface processing equipment and flowlines are installed, leaving time for bacteria to colonize. Once bacteria become established in a well, they develop biofilms that supply a stream of bacterial contamination downstream the well through water tanks, flow lines and disposal facilities. Biofilms protect the bacteria from the chemical biocides and a program of regular, high volume biocide application must be initiated merely to keep the free-swimming bacteria in check and minimize problem bacterial byproducts. Biofilms themselves are impervious to biocides, and can only be mechanically scoured, as with pipeline “pigs”. In addition, there is increasing biocide resistance being observed in hydro-fracture and flood water bacteria.
Other reservoirs are “flooded” with water to enhance recovery—usually oil recovery. In “water flood” operations, injection wells are drilled into the producing horizon and water is pumped—as in fracturing—to displace the oil and/or gas through a formation into other “recovery” well(s) in the same field. Since the water is injected into the reservoir is contaminated with bacteria, similarly to the water used for “fracing,” the same problems of souring, fouling, and corrosion occur.
Bacteria also cause a host of additional problems in other sectors of the petroleum industry. Another potential “expense” is the social cost of catastrophic failure. Microbiologically-induced corrosion (MIC) has been a factor in several major oil and gas pipeline incidents, including the well-publicized 2006 Alaska Pipeline spill. MIC occurs on the insides of pipes or storage vessels, and especially under biofilms.
A better control strategy would be: inexpensively manufactured, environmentally benign, adaptable to changing microorganisms to prevent resistance, targeted towards those microorganisms that constitute the threat, and capable of penetrating and destroying biofilms. Such a control strategy would optionally be able to sense and adjust to the different concentrations of microorganisms encountered, even within the well. The present invention is just such a strategy, providing bacterial control based on bacteriophages, the natural predators of bacteria.